This invention relates in general to magnetic brush developer apparatus for electrographic reproduction equipment, and more particularly to magnetic brush developer apparatus having an agitated developer nap bounded by a non-agitated, sealing, developer nap.
In the electrographic process for making reproductions of input information, an electrostatic charge pattern is formed on the surface of an insulating member in image-wise configuration corresponding to the information to be reproduced. The charge pattern is developed by applying developer material to such pattern to form a visible image. The visible developer material image is then either transferred to a receiver member and fixed to such member, or fixed to the insulating member itself. In commercial high speed electrographic reproduction equipment, a common mechanism for developing electrostatic charge patterns is a magnetic brush developer apparatus.
A typical magnetic brush developer apparatus includes one or more magnets located within an applicator member. The applicator member may rotate about fixed magnets, the magnets may rotate wtihin a fixed applicator member, or both may rotate in the same or opposite directions. Developer material comprises, for example, a mixture of finely divided pigmented thermoplastic marking particles (toner) held to the surface of ferromagnetic particles (carrier) by electrostatic charges created by triboelectrification. The carrier particles, with the attached toner particles, are held on the applicator member in a bristle-like formation by the magnetic fields of the magnets to form a brush nap. Of course, developer material of the type comprised solely of marking particles which exhibit magnetic properties (referred to as single component developer) is also suitable. The developer material is then brought into contact with the electrostatic charge pattern by brushing the nap bristles across the surface of the insulating member. When the developer material contacts the surface of the insulating member, the electrostatic attraction for the triboelectrically charged toner particles by the charge pattern on the insulating member overcomes the attraction of the carrier particles for the toner particles (or the magnetic attraction for single component developer) and the pattern is developed; see, for example, U.S. Pat. No. 3,703,395, issued Nov. 21, 1972 in the name of Drexler et al.
Due to the fact that the developer material is presented to the charge pattern in that segment of the nap bristles contacting the insulating member, only a portion of the total amount of material in the bristles is available to carry out development. In most instances sufficient developer material is available for complete charge pattern development; however when the insulating member is moving at a relatively high speed through the developer material, or when the iunsulating member has a large area of dense charge patterns for example, there may be insufficient material in the segment of the bristles contacting the charge pattern for complete development. This results in defects in the developed image such as washed out areas or white streaks or spots in the reproduction. One arrangement for increasing the available developer material is shown in the Research Disclosure Bulletin of August 1982 at p. 299 (No. 22010). In this arrangement, the nap bristles on an applicator shell are agitated throughout the development zone by counter rotating magnets adjacent to the development zone. Such agitation causes the bristles to tumble or flip so that an increased amount of developer material is available for charge pattern development; however, it may also result in the escaping of some particles from the confinement of or positional control of the magnetic fields, which particles have the potential for contaminating the interior of the reproduction equipment.